Monovalent metal cation complexes, stability

They form relatively stable complexes with monovalent cations, especially with alkali metal cations, where the complex stability has ion-selective character [88, 100, 193, 202, 213]. 

Both monensin (24) and nigericin (25) complex Na+ and K+ strongly but not selectively. The crystal structures of the Na+, Tl+ and Ag+ complexes all show the metal ion to be in an O-rich cavity. The carboxylate group is not involved however.97 With the antibiotics (26), (27) and (28) the thermodynamic stabilities (Table 9) are greater for the divalent than for the monovalent metal ions.98 The conformations adopted in these complexes axe very solvent dependent, and the implication of these to the biological transportation of the cations has been discussed.99... 

The conformational studies on the ferrichromes and on ferrioxamine B indicate that a number of intramolecular hydrogen bonds are formed in the process of metal-chelation and that these contribute to the overall stability of the coordinated conformation relative to that of the free species. Consistent with this view, it should be mentioned that extensive hydrogen bonding has also been observed in the low molecular weight monovalent cation complexes of the antibiotics monensin, nigericin, dianemycin, the enniatins and valinomycin by NMR spectroscopy (69, 70), X-ray crystallography (71, 72, 73), or both. Like the siderochromes, these compounds act by mediating cation fluxes across membranes. 

For the current discussion, RVS analysis has been carried out on cation-jt complexes of benzene-metal complexes. An overview of the results obtained for the RVS analysis carried out at the HF/6-31G level on the cation-jt complexes of benzene with monovalent and bivalent metal ions such as LP, Na, K, Mg and Ca + is presented in Figure 15.5. Generally, cation- t complexes are held by pure electrostatic effects due to strong attraction between opposite charges. Hence, bivalent metal ions show stronger interaction than the monovalent metal ions. However, the contribution of ion-quadrupole interactions in complex stabilization is still being... 

Basically, the same dependencies of the stabilization energies on increasing atomic numbers of metal cations are observed in metal-purine-pyrimidine complexes as in previously published metal-purine species (Burda et al. 1996). Stabilization energies of complexes with divalent ions are larger than those of monovalent ions, and M-GC stabilization energies are larger than those for M-AT complexes. Both conclusions reflect the dominant role of the ion-dipole electrostatic contribution to the stabilization energy of these complexes. 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

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